Field of the Invention
The present invention relates to a fixing device suitable for an image forming apparatus that forms an image on a recording medium using, for example, an electro-photographic system, and a method of controlling the fixing device. The present invention also relates to an image forming apparatus with a fixing device, such as an electro-photographic copying machine, a laser beam printer, a facsimile machine, or the like.
Description of the Related Art
As a fixing device mounted on an electro-photographic image forming apparatus, the configuration having a heater, a film (rotating unit) that is rotated while being heated in contact with the heater, and a pressure roller (pressure member) that is rotated while pressing the film is known. In this configuration, a recording material bearing an unfixed toner image (developer image) is heated while being nipped and conveyed at a fixing nip portion formed by the film and the pressure roller, thereby fixing the image on the recording material.
Here, it is ideal that all of the unfixed toner image on the recording material is fixed by being properly heated and melted. When there exists toner that is not dissolved by heat, however, toner that is dissolved too much, or toner that is electrostatically attached to the pressure roller or the film, such toner is transferred to the pressure roller or the film, and the toner that has been transferred to the film is further transferred to the pressure roller between sheets.
When the fixing operation is repeated in this state, the toner transferred to the pressure roller accumulates. When the accumulated toner exceeds a predetermined accumulation amount, the toner on the pressure roller adheres to the back surface of a subsequent recording material, thereby generating conspicuous toner contamination on the back surface of the recording material.
Therefore, in Japanese Patent Application Laid-Open No. H11-344894, the configuration is proposed in which a discharge control is performed to transfer the toner on the pressure roller to the film by heating the film until the film reaches a temperature equal to or greater than the softening point of the toner with the film being stopped after the completion of the fixing operation. By performing such discharge control, the pressure roller can be cleaned, and toner contamination on the back surface of the recording material can be suppressed.
In the configuration disclosed in Japanese Patent Application Laid-Open No. H11-344894, however, when the film is continuously heated with the film being stopped, the temperature rises greatly only in the fixing nip portion that is in contact with the heater, and the temperature of the portion other than the fixing nip portion does not largely change from the ambient temperature. As described above, when the pressure roller is suddenly driven in a state in which a temperature difference is generated between the fixing nip portion and the other portion in the rotation direction of the film, the film is deformed, causing a risk of generating a dent mark, as described below.
FIGS. 27A and 27B are schematic views of a film for explaining the mechanism of deformation of the film. FIG. 27A is a diagram showing a state in which the temperature of the heater is raised with the film being stopped (non-rotating state). FIG. 27B is a diagram showing the case in which the film is driven to rotate by rotating the pressure roller from the state shown in FIG. 27A.
As shown in FIG. 27A, when the temperature of the heater is increased with the film being stopped, the film in the vicinity of the fixing nip portion (broken line portion) locally thermally expands and the other portion (solid line portion) does not thermally expand. For this reason, thermal stress is applied in the vicinity of the boundary between the portion that has thermally expanded and the portion that has not thermally expanded in the rotation direction (circumferential direction) of the film, and distortion occurs in the film. As the temperature difference between inside the nip and outside the nip of the film increases, the amount of distortion increases due to the difference of expansion amount.
Next, as shown in FIG. 27B, when the film rotates with a thermal stress being applied, the film is pulled by the pressure roller, and the stress is further concentrated near the boundary between the portion that has thermally expanded and the portion that has not thermally expanded, thereby permanently deforming the film, causing a dent mark to generate.
When the fixing process is performed with a dent mark, the film surface does not contact the recording material at the dent mark portion, so that heat is not transferred to the toner and the fixing becomes insufficient, thereby generating image failure, such as a whitened out image. Such image failure is remarkably generated particularly in a low temperature environment in which securing of fixing ability is relatively difficult. Also, if the film is continuously used with the dent mark, the bending of a dent mark may be repeated many times and the film may crack.